1. Field of the Invention
The present invention relates to a printing apparatus and a laser diode driving apparatus suitable for use with a laser printer or the like.
2. Description of the Prior Art
U.S. Pat. No. 4,492,482, for example, describes a printer for printing an image on a printing paper. Also, there is proposed a printing machine (printer) called a laser printer.
The conventional laser printer emits a laser beam modulated by an image signal to scan a mirror called a polygon mirror. A laser beam reflected by the polygon mirror is irradiated on an organic photoconductor (i.e., OPC) sheet to render the OPC a conductivity corresponding to the intensity of the laser beam, whereafter an electrostatic latent image, i.e., an electrostatic electrified pattern having a pattern corresponding to light and shade of an original image formed on the OPC is formed on the surface called an OPC sheet by the discharge of the OPC. A toner is put on the surface of the OPC with electrification on which this electrified pattern is formed. Further, the OPC with the toner and a printing paper are closely contacted. Under this condition that the OPC and the printing paper are held close to each other, they are heated and pressed to put the toner corresponding to original image data on the printing paper with electrification and then the toner is fixed onto the printing paper. Thus, the original image data is printed (or set) on the printing paper.
Although it is customary that such laser printer obtains a printed image according to binary information of white or black, according to the recent laser printer, a signal supplied to a driving unit that drives a laser diode to emit a laser beam is modulated to provide a pulse width modulated (PWM) signal to thereby obtain a middle tone in a pseudo gray scale fashion.
The above-mentioned laser printer using the OPC is produced very expensively because the OPC is expensive. Also, the mechanism thereof and the circuit scale thereof become complex and enlarged.
According to the above-mentioned method of supplying the pulse width modulated signal to the driving unit in order to obtain an intermediate tone, the signal can be modulated only in the beam scanning direction and the signal cannot be modulated in the planar direction. Therefore, when the diameter of the beam spot is reduced to enlarge the spacing between the scanning lines, the spacing between the scanning lines becomes a so-called unprinted portion. There is then the disadvantage that density of a printed image cannot be increased.